Fix extent_quantize() to handle greater-than-huge-size extents.
Fix extent_quantize() to handle greater-than-huge-size extents.

Allocation requests can't directly create extents that exceed
HUGE_MAXCLASS, but extent merging can create them.

This fixes a regression caused by
8a03cf039cd06f9fa6972711195055d865673966 (Implement cache index
randomization for large allocations.) and first released in 4.0.0.

This resolves #497.

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc